Another change from earlier models is the toothed ABS ring on the front CV joints and rear hubs has been replaced by ferromagnetic rings built into the “active” wheel bearing oil seals. The clearance between the wheel sensor and the ferromagnetic ring on the wheel bearing is critical excess clearance due wheel bearing wear can trigger the system to log an ABS fault.

Because of these changes fault finding on this ABS system without dealer level diagnostic equipment requires a more careful approach than with earlier types of ABS as both the Hall Effect sensors and active wheel bearings are more easily damaged. Hall Effect sensors on this model should not be tested by measuring resistance as a simple resistance test with an average Digital Multi Meter may burn out the sensor.

To avoid wasted time, unnecessary expense and frustration please read this HowTo through to the end before testing replacing and components.

Start-up Self Test

(1)Start-up test: at ignition switch on the ECU in the ABS Modulator performs a basic self test during which the yellow ABS illuminates. During the start-up test each sensor is tested electrically and the ABS modulator is tested both electrically and hydraulically. If the ABS light remains on it most likely points to an electrical fault or less likely an ABS modulator or hydraulic fault.

(2)In normal operation each wheel sensor is supplied with a 12v supply, if a sensor fails start-up self test the 12v supply to that sensor is automatically shut-down by the ABS ECU.

(3)If this first stage of the self-test is passed the ABS warning light extinguishes and the next stage of self-test continues when the vehicle is in motion.

(4)If the ABS fault warning light extinguishes as normal but then re-illuminates after driving a short distance (over 7.5 mph) then it usually suggest to the ABS having detected implausible signal from one of the wheel sensor.

(5)An implausible signal from a wheel sensor would usually be either a problem with an active wheel bearing or intermittent sensor or electrical connection.

(6)Another common cause of the ABS illuminating on an implausible wheel sensor error is mismatched tyre sizes, of course this will not show up on an oscilloscope test.

Important Clues on Where to Start

The ABS wheel sensors signals are passed by by the ABS Module ECU to other systems on the car this can give pointers to where start test individual sensors.

(1)Speedometer is driven by the signal from the right front wheel sensor, if the speedometer works normally the fault is less likely to be in the right front wheel sensor or associated parts.

(2)On KV6 models with cruise control the speed signal to the cruise control system is taken from the front left wheel speed sensor, If the cruise control works normally the fault is less likely to be in the right front wheel sensor or associated parts. The engine ECU also takes its' speed signal from the left front sensor.

(3)SatNav system takes data from both rear wheel sensors, if ABS and SatNav faults appear at the same time it is likely to be a problem related to a rear sensor.

Before testing individual wheel sensors it is worth removing the car's battery and battery box and unplugging the multi-pin connector from the ABS Modulator and checking for water ingress and corrosion, this is a known problem on Rover 75's particularly pre-Longbridge cars.

Premature ABS Activation

Premature ABS activation or unexpected pulsing felt through the brake pedal can be caused by problems with the the ferro magnetic rings in the wheel bearings unlike older system where a clean up and visual inspection of the toothed reluctor rings with active wheel bearings will often reveal the cause of the problem the only way diagnose this type of problem is to examine the signals from each sensor with an oscilloscope while the vehicles is driven preferably at constant speed. However with this type of fault it is worthwhile checking all the electrical connections and bleeding the brake hydraulics.

Testing an Active Wheel Sensor

The Hall Effect wheel sensors should not be tested by measuring resistance as it is likely to burn out the sensor.

Each sensor is connected to the ABS modulator by two wires, one is a nomminal 12 volt supply the other the signal return to the ABS modulator. Following a visual inspection of the wiring and connectors the first test is to measure voltage present between each of these two sensors wires and a good ground on the vehicle body.

Because the wheel sensor connectors are very difficult to back probe to get usable test connections it is more convince to break into the wiring of the vehicle either by making tap connections or making a special breakout connector using plug & socket parts salvaged from a scrap vehicle.

ABS Wiring Colour Codes

Wheel Sensor

12v Supply

Signal Output

Notes

Front Left

Red, Black stripe

Red

Front Right

Yellow

Yellow, Black stripe

Also Drives Speedometer

Rear Left

Green,

Green, Black stripe

Rear Right

White

White, Black stripe

Wheel Sensor DMM Test.

Tools required Digital Multi-Meter, Car Jack.

1.Jack up the car so that the wheel is clear of the ground and can be rotated by hand.

2.Turn ignition and allow ABS system four seconds to complete self-test.

3.Using a Digital Multi Meter (DMM) set to the 20v DC range, connect the negative lead of the meter to a good earth ground on the vehicle body then check the voltage on the 12v supply wire to the sensor.If a +12v supply is present then the sensor has passed power on self test.If the +12v supply is not present then it points to a failure in the sensor or the wiring between the sensor and the ABS modulator ECU. Connection problems due to water ingress by capillary action via the wiring from the front ABS sensor are not unknown on the 75/ZT, see notes in a later section.

4.Now with the meter on the 2v dc range check the voltage on the sensor output wire, expect this to read between approximately +0.6 volts and +1.7 volts.

5.With the wheel clear of the ground rotate the wheel by hand while checking output voltage which should switch between approximately between +0.6v and +1.7v when the wheel is rotated.

6.Repeat for each in turn wheel.

Passing this simple test will only show if the wheel sensor is energised and switching in response to movement of the active wheel bearing it will not indicate if the active wheel bearing is generating plausible signal. As the magnetic ring in the wheel bearing is relatively delicate and the clearance between the ring and the sensor is critical, failure is relatively common.

An oscilloscope is the only way to check the quality of the signal from a sensor, the output wave form should be examined using an oscilloscope when the vehicle is being driven on the road.

Oscilloscope Test on Signal from Wheel Sensor

This test is to check the quality of the signal returned to the ABS Modulators ECU by a wheel sensor. Any defect in the wheel bearing magnetic ring could generate an irregular signal which will fail the plausibility test and toggle the ABS light on as soon as the vehicle speeds exceeds 7mph.

As with the DMM test the measurements are with the ignition on and power on self-test complete, the oscilloscope connected between the the sensor signal output wire and a good earth the vehicle body.

To obtain meaningful results I would suggest the test is carried out when the vehicle is on the road driven at constant speed.

When the car is being driven the wheel sensor output viewed on an oscilloscope should be a nice regular square wave between a low of +0.6 and and peak of +1.6v. I would suggest starting with an oscilloscope timebase setting of 0.1 seconds per division.

Replacing Wheel Sensors

Take care to only purchase OEM quality sensor either MG-Rover/X-Part or Hella cheap unbranded sensors are an expensive lottery.

The sensors are held in by single 6mm diameter bolts with 8mm hex heads, so once you have identified a sensor you think is defective it should be simple to replace, well no :-(

The bolts usually come free easily enough but the sensor bodies made of a relatively fragile plastic are an extremely tight fit in the housing even when new.

The rear sensors are easiest to remove. If after soaking with WD40 they show no signs of budging then simply remove the brake disc and wheel hub to gain access to the working end of the sensor, then use a 10 or 12mm diameter drift to punch it through from brake shoe back plate side.

The front sensors are generally much more difficult to remove and attempt to remove a sensor by pulling with mole grips is likely to result in the sensor breaking up leaving the stub firmly seized in the hole leaving a choice of three options:

1. Carefully drill the sensor out with a 12mm drill, requires great care to avoid damaging the active wheel bearing also results in iron oxide, metallic dust and plastic swarf trapped in the void between the CV joint and the wheel bearing which can often cause contamination problems with the active wheel bearing.

2.Undo the outer CV joint retaining nut, bottom ball joint, track rod end and anti-roll bar link and pop the CV joint out from the hub. This allows the sensor to be drifted out.

3.Replace the complete swivel hub with a scrapyard part either with a known good sensor or fitted a new sensor. You can connect the replacement sensor parts up to the car and do a quick DMM test and self-test on the sensor and active wheel bearing before reassembly. Because of problems with active wheel bearings replacing the complete swivel hub with a tested item has become my preferred option especially on higher mileage vehicles.

Notes on Active Rear Wheel Bearings Replacement.

The rear wheel bearings come complete as part the wheel hub assembly and is very easy to change however take care to carefully examine the replacement assembly before fitting.

As the clearance between the wheel sensor and the ferromagnetic ring on the wheel bearing is critical any excess clearance or run out of the ferromagnetic ring is likely to cause problems with the signal from the sensor.

When you unpack the new part carefully check that the ferromagnetic ring as been pressed in evenly and is sitting flush with the rim of the hub.

The front wheel bearings are a fairly major job during which you can run into niggling snags, to minimise time off the road I would recommend buying a second hand swivel hub and drive flange assembly and either fitting as it comes or overhauling it with a new bearing before fitting.